When matter is continually supplied with energy, its temperature increases and it typically transforms from a solid to a liquid and, then, to a gaseous state. Continuing to supply energy causes the system to undergo yet a further change of state in which neutral atoms or molecules of the gas are broken up by energetic collisions to produce negatively charged electrons, positive or negatively charged ions and other species. This mix of charged particles exhibiting collective behaviour is called “plasma”, the fourth state of matter. Due to their electrical charge, plasmas are highly influenced by external electromagnetic fields, which make them readily controllable. Furthermore, their high energy content allows them to achieve processes which are impossible or difficult through the other states of matter, such as by liquid or gas processing.
The term “plasma” covers a huge range of systems whose density and temperature vary by many orders of magnitude. Some plasmas are very hot and all their microscopic species (ions, electrons, etc.) are in approximate thermal equilibrium, the energy input into the system being widely distributed through atomic/molecular level collisions; examples include flame based plasmas. Other plasmas, however, particularly those at low pressure (e.g. 100 Pa) where collisions are relatively infrequent, have their constituent species at widely different temperatures and are called “non-thermal equilibrium” plasmas.
In these non-thermal equilibrium plasmas, the free electrons are very hot with temperatures of many thousands of Kelvin (K) whilst the neutral and ionic species remain cool. Because the free electrons have almost negligible mass, the total system heat content is low and the plasma operates close to room temperature thus allowing the processing of temperature sensitive materials, such as plastics or polymers, without imposing a damaging thermal burden. The hot electrons create, through high energy collisions, a rich source of radicals and excited species with a high chemical potential energy capable of profound chemical and physical reactivity. It is this combination of low temperature operation plus high reactivity which makes non-thermal equilibrium plasma technologically important and a very powerful tool for manufacturing and material processing as it is capable of achieving processes which, if achievable at all without plasma, would require very high temperatures or noxious and aggressive chemicals.
For industrial applications of plasma technology, a convenient method is to couple electromagnetic power into a volume of process gas, which can be mixtures of gases and vapours in which the workpieces/samples to be treated are immersed or passed through. The gas becomes ionised into plasma, generating chemical radicals, UV-radiation, and ions, which react with the surface of the samples. By correct selection of process gas composition, driving power frequency, power coupling mode, pressure and other control parameters, the plasma process can be tailored to the specific application required by a manufacturer.
Because of the huge chemical and thermal range of plasmas, they are suitable for many technological applications. These properties provide a strong motivation for industry to adopt plasma-based processing, and this move has been led since the 1960s by the microelectronics community which has developed low pressure Glow Discharge plasma into an ultra-high technology and high capital cost engineering tool for semiconductor, metal and dielectric processing. The same low pressure Glow Discharge type plasma has increasingly penetrated other industrial sectors since the 1980s offering, at more moderate cost, processes such as polymer surface activation for increased adhesion/bond strength, high quality degreasing/cleaning and the deposition of high performance coatings. Thus, there has been a substantial take-up of plasma technology. Glow discharges can be achieved at both vacuum and atmospheric pressures.
Atmospheric pressure plasmas, however, offer industry open port or perimeter systems providing free ingress into and exit from the plasma region by workpieces/webs and, hence, on-line, continuous processing of large or small area webs or conveyor-carried discrete webs. Throughput is high, reinforced by the high species flux obtained from high pressure operation. Many industrial sectors, such as textiles, packaging, paper, medical, automotive, aerospace, etc., rely almost entirely upon continuous, on-line processing so that open port/perimeter configuration plasmas at atmospheric pressure offer a new industrial processing capability.
Corona and flame (also a plasma) treatment systems have provided industry with a limited form of atmospheric pressure plasma processing capability for about 30 years. However, despite their high manufacturability, these systems have failed to penetrate the market or be taken up by industry to anything like the same extent as the lower pressure, bath-processing-only plasma type. The reason is that corona/flame systems have significant limitations. They operate in ambient air offering a single surface activation process and have a negligible effect on many materials and a weak effect on most. The treatment is often non-uniform and the corona process is incompatible with thick webs or 3D webs while the flame process is incompatible with heat sensitive powdered particles.
Considerable work has been done on the stabilisation of atmospheric pressure glow discharges, such as described in Okazaki et al. J. Phys. D: Appl. Phys. 26 (1993) 889-892.
Further, U.S. Pat. No. 5,414,324 describes the generation of a steady-state glow discharge plasma at atmospheric pressure between a pair of electrically insulated metal plate electrodes spaced up to 5 cm apart and radio frequency (RF) energised with a root means square (rms) potential of 1 to 5 kV at 1 to 100 kHz.
Metal oxides and metalloid oxides are made by a wide variety of processes. Titanium dioxide for example may be made by mixing titanium ores in sulphuric acid to make titanium sulphate, which is then calcined to produce titanium dioxide. Silicon dioxide or titanium dioxide may be prepared by reacting their respective chloride with oxygen at an elevated temperature. In this method, the reactants are brought to reaction temperatures by combusting a flammable gas such as methane or propane.
One of the main problems with the “wet chemistry” type preparations of oxides is that the average particle size of the resulting powder particles tend to be significantly larger than optimally required in many of today's applications for such products.
The use of thermal-equilibrium plasma processes for the production of the oxides of silicon, titanium, aluminium, zirconium, iron and antimony has been described in US 20020192138, which was published after the priority date of the present application, in which a plasma generator producing a temperature of between 3000 and 12000° C. is used to oxidize vapours of salts of the above metals and metalloids.
Many electronics and/or optical based applications exist for metal and metalloid oxides, for example, they may be utilized to enhance the refractive indices of silicone polymers, organic resins and glasses such as by blending TiO2 or ZrO2 with silica or organopolysiloxane or to react silica or silicone/silicate precursors with titanium alkoxides as described in WO 99/19266 or with a TiO2—ZrO2—SiO2—SnO2 composite sol as described in JP 11-310755. However, the refractive index of the final inorganic material is usually lower than theoretically expected either because of the difficulty of preparing nano-sized particles, the inhomogeneity resulting from a broad particles size distribution, the tendency for nanoparticles to self-aggregate resulting to a light scattering effect phenomenon.
Organosilicone resins are generally synthesized by the hydrolysis and subsequent condensation of chlorosilanes, alkoxysilanes and silicates, such as sodium silicate. They are generally described using the M, D, T and Q nomenclature in which M units have the general formula R3SiO1/2, units have the general formula R2SiO2/2, T units have the general formula RSiO3/2 and Q units have the general formula SiO4/2 where, unless otherwise indicated, each R group is an organic hydrocarbon group, typically a methyl group.